Javascript函数返回＆＃39; NaN＆＃39;或者＆＃39; undefined＆＃39;

Question

我有以下代码：

    getLastInsertId = function(collection, field){

    field = (typeof field === "undefined") ? '_id' : field;


    var qry = JSON.parse('{"fields" :{"'+field+'": 1}, "sort" : {"$natural": -1}, "limit": 1}');
    //console.log(qry);
    var lastInsertId = collection.find({}, qry).fetch()[0];

    if(lastInsertId === 'undefined'){
        //var x = 1;
        return "1";
        //console.log(Number(1));


    }else{
        //return (+lastInsertId + 1);

    }



};

如果我使用以下内容，则返回NaN：

return 1;

有人可以指导我，我做错了什么以及如何纠正。

Answer 1

试试这个

var qry = {"fields" :{"'+field+'": 1}, "sort" : {"$natural": -1}, "limit": 1};
//console.log(qry);
var lastInsertId = collection.findOne(qry);
lastInsertId = lastInsertId._id;